This invention relates to abrasive articles in which a thin layer or, more usually, a single layer of abrasive grit is adhesively bonded to a backing. Such products are coated abrasives, more commonly referred to as “sandpaper,” and fibrous abrasive pads, for example. Conventionally, glue or thermosetting resins such as phenol formaldehyde resins (“PF resins”) and urea formaldehyde resins (“UF resins”) have been used in coating formulations. UF resins, which are less expensive than phenolic resins, have been used to reduce the cost in abrasive products. However, current UF resin systems used in the manufacture of coated abrasive products do not provide desirable working times and curing times under ambient or near ambient conditions. The coated abrasive industry is seeking time released curable urea formaldehyde binder compositions in order to have enough working time, yet faster curing to provide a sharp image and non-curled sand paper.
Phenolic resin compositions are known that can cure at ambient temperatures with rapid cure at modestly higher temperatures. Such resins, as in U.S. Pat. No. 5,296,520 to Gerber, provide controlled work time for hardening phenolic resins at ambient temperature using aryl phosphite latent acid catalysts. However, Gerber found that urea and other amide compounds were very effective retarders of ambient temperature hardening of the resin with the aryl phosphite hardening agents.
Nevertheless, UF resins are different from phenolic resins and each have vastly different chemistry and methods of manufacture. UF resins are prepared by a condensation whereby the nitrogen of urea reacts with the carbonyl group of formaldehyde. In contrast, an exemplary phenolic polymer is a phenol formaldehyde resin prepared by aromatic substitution of the multiple activated sites of a phenol, initially by formaldehyde, followed by reaction with other reactive intermediates. In the preparation of phenolic resins, catalysts such as strong base, zinc acetate, and borates can be used to provide phenolic resoles, while strong acid can be used to provide phenolic novolacs. UF resins can be prepared using a strong acid catalyst, such as, for example, sulfuric acid. Structurally, phenolic resins are highly aromatic, and contain aromatic end groups. In contrast, UF resins are amino resins and essentially non-aromatic. Regarding curing properties, phenolic polymers range from thermosetting resoles to thermoplastic novolacs. UF resins are thermosetting resins. Accordingly, PF resin chemistry is not predictive of UF resin chemistry, as shown by Gerber, op.cit.
UF precondensates can be blended with liquid phenolic resin systems. However, such systems are catalyzed by providing a basic environment (pH above 7), in contrast to the acid catalyst used for unblended urea formaldehyde prepolymers.
Accordingly, curable urea formaldehyde binder compositions employing a latent acid hardening component are needled that will provide desirable working times and curing times under ambient or near ambient conditions. The results presented below provide surprising and unexpected improvements in urea formaldehyde based binder compositions.